Modern vehicles are increasingly equipped with a variety of active safety systems, such as active cruise control systems and lane keeping systems, that are intended to supplement driver control of the vehicle when necessary to maintain safe operating conditions. One circumstance in which it may be desirable to enable or disable certain active safety systems occurs when the driver takes his or her hands off of the steering wheel of the vehicle.
Various methods and systems have been proposed to determine whether the driver's hands are in contact with the steering wheel. Some steering wheels have been equipped with capacitive or pressure sensors intended to detect the presence of the driver's hands on the steering wheel. The modified steering wheels are expensive, however, and can be unreliable. Other methods and systems have relied on measurements of steering torque and/or steering angle as indicators of the presence or absence of the driver's hands. These methods and systems cannot, however, account for circumstances in which steering is not normally required. For example, when a vehicle is travelling in a straight line at a relatively constant speed, the driver's hands may remain on the wheel, but not be involved in steering and therefore fail to generate the changes in steering torque/angle necessary to determine whether the driver's hands are, in fact, in contact with the steering wheel. Still other methods and systems introduce a vibration of the steering wheel and measure the response which differs depending on whether the driver's hands are on the steering wheel. It is desirable to produce very low levels of vibration during these assessments so that the vibration is not detected by the driver. This latter approach has proven difficult to implement, however, due to the excessive levels of vibration that need to be produced for adequate measurement and interpretation. In particular, if the introduced vibration is too small, it is difficult to directly identify a response dictated by the presence or absence of the driver's hands as opposed to a response resulting from other vehicle operating conditions. Increasing the level of vibration, however, will render the vibration detectable to the driver creating an undesirable distraction.